Ionization chamber for ion beams and method for monitoring the intensity of an ion beam

ABSTRACT

The invention relates to an ionization chamber for ion beams and to a method of monitoring the intensity of an ion therapy beam by way of such an ionization chamber. For that purpose, the ionization chamber includes a chamber housing, a beam inlet window and a beam outlet window, a chamber volume filled with counting gas, a high-voltage anode and a high-voltage cathode, wherein the ionization chamber is constructed flat and sandwich-like from plate-shaped large-surface-area structures of the above-mentioned components, which are aligned orthogonally relative to the axis of the ion beam, and a centrally arranged large-surface-area orthogonally aligned plate-shaped counting anode is surrounded on both sides by a large-surface-area plate-shaped high-voltage cathode consisting of two parallel cathode plates, and the chamber housing consists of a housing frame, which frames a square ionization chamber volume, and on which frame the beam inlet window and the beam outlet window are mounted gas-impermeably.

TECHNICAL FIELD

The invention relates to an ionisation chamber for ion beams and to amethod of monitoring the intensity of an ion beam according to thepreamble of claims 1 and 16.

BACKGROUND OF THE INVENTION

Such ionisation chambers are also known in the prior art as ion countingtubes. The chamber housing is customarily made from a tube, with one ofthe two end portions of the tube serving as the beam inlet window andthe other end of the tube serving as the beam outlet window. The tube isfilled with a counting gas under reduced pressure and has a cylindricalhigh-voltage cathode that lies coaxially in the counting tube, insulatedfrom the tube wall. Located in the centre of the tube is a cylindricalhigh-voltage anode which is insulated from the high-voltage cathode andthe surrounding tube. In order to operate the ionisation chamber, avoltage is applied between the high-voltage cathode and the high-voltageanode and the current between the cathode and anode is measured. Ifcharged particles, such as ions, are passed through the ionisationchamber or are captured by the ionisation chamber, the current betweenthe cathode and the anode increases in dependence upon the number ofions that pass through the ionisation chamber. More complex cylindricalionisation chambers have a large number of axially aligned anodes inorder, for example, to measure the path of a charged particle or ionthrough a tubular ionisation chamber by means of anodes distributedaxially over the cross-section.

A disadvantage of such cylindrical ionisation chambers is their largeaxial dimension and the relatively complex construction of the countinganodes. Moreover, the space required by such ionisation chambers in thedirection of the diffusion of a beam is relatively large. The spaceavailable at the beam outlet in a treatment room in front of patientsis, however, very limited. Moreover, with therapy systems in which anion beam is scanned over the entire extent of a tumour tissue, theremust be available an ionisation chamber of hitherto unknown dimensionsin terms of breadth and length. Generally, all the measurements in thebeam must be carried out in front of the patient in the transmissionmode. It is imperative to avoid impairment of the quality of the beam,for example as a result of projectile fragmentation and angular scatterof the beam particles.

SUMMARY OF THE INVENTION

The problem underlying the invention is accordingly to provide anionisation chamber for ion beams and a method of monitoring theintensity of an ion therapy beam that overcomes the disadvantages ofexisting ionisation chambers, is suitable for monitoring and controllingpatient irradiation in the context of tumour therapy with heavy ionsthat are concentrated with high energy into a pencil beam, in which thedimensions of the detector in the direction of the beam are small, thatenables a high level of safety to be achieved, especially in respect ofplasma and spark formation, and can be used in the field of medicine.

For that purpose, the ionisation chamber for ion beams consists of achamber housing, a beam inlet window and a beam outlet window, a chambervolume filled with counting gas and a high-voltage anode and ahigh-voltage cathode. The ionisation chamber is constructed flat andsandwich-like from plate-shaped large-surface-area structures of thosecomponents, which are aligned orthogonally relative to the axis of theion beam. A centrally arranged large-surface-area orthogonally alignedplate-shaped counting anode is surrounded on both sides by alarge-surface-area plate-shaped high-voltage cathode consisting of twoparallel cathode plates. The chamber housing consists of a housing framewhich frames a virtually square ionisation chamber volume, and on whichframe the beam inlet window and beam outlet window are mountedgas-impermeably. Such a device has the advantage of being easier tomaintain since the plate-shaped construction can be removed from thehousing frame and replaced by simply dismantling or removing differentplate structures. The plates can be replaced easily and suitable numbersthereof can be held in stock. The plate-shaped structure also enablesmass production of spare parts and finished ionisation chambers.

In a preferred embodiment of the invention, the counting gas is a gasmixture of argon or krypton and carbon dioxide, preferably having a gasvolume mixing ratio of 4:1, which is adapted to the energy and intensityof the ion beam, and is introduced into the ionisation chamber. Acounting gas of such composition has the advantage, compared withcustomary air-filled cylindrical ionisation chambers, that themeasurements are easier to reproduce since in this case air humiditydoes not influence the sensitivity of the ionisation chamber. Such acounting gas ensures a good signal/noise ratio and makes available ahigh dynamic range in the particle rates. With the preferred countinggas, sufficient dielectric strength is also ensured.

Such a counting gas is preferably of the highest purity since the signalsensitivity, especially in its amplitude and waveform, is impaired byimpurities. Moreover, inside the chamber volume there are preferablyused for the individual plate elements and other supporting andinsulating elements as well as for auxiliary units and sensors materialsthat do not release gases, or elements and components that do releasegases are cast in epoxy resin.

In a further preferred embodiment, the ionisation chamber has sensorsthat are mounted in the housing frame in through openings that aregas-impermeably sealed and that measure the counting gas pressure andthe counting gas temperature. The ionisation chamber is operated withslightly elevated pressure compared with the ambient air, whichadvantageously makes penetration of extraneous gases more difficult. Forthat purpose, the extent of the gas reflux from the chambers ismonitored by a sensor system in the counting gas outlet region or in theoutlet. By measuring gas pressure and gas temperature, it isadvantageously possible to monitor the gas density and, if necessary,keep it constant, the gas density being used directly in thedetermination of the ion beam particle number.

The beam inlet window and the beam outlet window, which aresubstantially square, preferably consist of radiation-resistantnon-polarisable plastics films. These are secured to metal plate-shapedframes, which in turn seal off the ionisation chamber volumegas-impermeably from the beam inlet window and from the beam outletwindow by means of O-ring seals in the housing frame. Thatgas-impermeable construction keeps impurities away from the counting gasand gas exchange of the chamber volume with the environment by diffusionis minimised even when the ionisation chambers are not in operation.

The beam inlet window and the beam outlet window preferably comprisepolyimide or polyester films, which has the advantage that exclusivelyradiation-resistant and non-polarisable materials are exposed to the ionbeam, so minimising the effect on the ion beam and the ion beamintensity.

In a further preferred embodiment of the invention, the beam inletwindow and the beam outlet window are metallised on the side facing theionisation chamber volume. Such metallisation of the beam inlet windowand the beam outlet window prevents the windows from becoming chargedand thus prevents falsification of the measurement values, since chargescan be conducted away to the ionisation chamber housing directly by wayof the metallisation of the windows and by way of the window frames. Theionisation chamber housing is thus advantageously earthed.

Such metallisation can be achieved by aluminising or nickel-plating oneof the sides of the beam inlet window or beam outlet window. Suchaluminised films have a conductive layer in order to avoid highelectrical field densities as trigger points for gaseous discharges, sothat the occurrence of gaseous discharges is minimised. Moreover,metallised films form a smooth surface which also serves to prevent highelectrical field densities at trigger points for gaseous discharges.

The large-surface-area plate-shaped counting anode and thelarge-surface-area plate-shaped high-voltage cathode preferably consistof mesh mounted in a frame which is supported against the housing framein an electrically insulating manner. In the process, electricallyinsulating spacing elements define the spacing between the countinganode and the high-voltage electrode. The use of meshes instead of filmsfor the anode and cathode has the advantage that it is possible tooperate with relatively large mechanical pre-stressing for the detectorplanes of mesh material. The uniformity of the signal across the siteand across the extent of the detector surface of the ionisation chamberis thus improved, which has an advantageous effect in particular in thecase of the relatively large chamber cross-sections in the edge regionsof the active volume of the chambers.

Compared with large-surface-area films as counting anode surfaces or ascathode surfaces, meshes have the advantage that there is less sagging,in particular when high voltage is applied, because of their higherpre-stressing capacity. Such sagging is caused by mutual electrostaticattraction of the films or mesh electrodes mounted parallel with oneanother. When meshes are used, however, the spacing of the electrodesfrom one another, especially in the centre, remains relatively constantso that the field densities can advantageously be kept locally constant.

Instead of using metal fibre mesh, it is preferable to use for thelarge-surface-area plate-shaped counting anode and thelarge-surface-area plate-shaped high-voltage cathode mesh made frommetal-coated plastics fibres. Such mesh made from composite fibres ofplastics and metal coating has the advantage that it is lighter and canbe loaded with relatively high pre-stressing and it has a low nuclearcharge number for the carrier filaments. The electrode function isprovided by the metal coating and a capacity to bear a high mechanicalload is achieved as a result of the plastics core of the fibres, thatcapacity to bear a high mechanical load in turn being a precondition forhigh mechanical pre-stressing of the high-voltage plate electrodes.

The large-surface-area plate-shaped high-voltage electrode and thelarge-surface-area plate-shaped counting anode are preferably made ofnickel-coated plastics mesh or nickel-coated polyester mesh. Thatcomposite material has the advantage not only that its plastics partconsists of radiation-resistant and non-polarised materials but alsothat it provides a smooth surface, which allows high electrical fielddensities without triggering gaseous discharges.

In a preferred embodiment, the counting gas is supplied into the regionof the ionisation chamber volume that is lowermost with respect togravity, and is discharged in the uppermost region. For that purpose,the housing frame of the ionisation chamber has a counting gas inletopening and a counting gas outlet opening. By means of that preferredembodiment, advantageously a laminar flow of counting gas through theionisation chamber can be provided by way of the counting gas inletopening and the counting gas outlet opening. Inside the chamber, thecounting gas is preferably guided through stainless steel tubes havingvariable outlet and inlet holes.

A gas flow sensor is preferably arranged outside the ionisation chambervolume in order to monitor the throughflow of counting gas, so as tokeep the chamber volume as small as possible. In addition to simplymonitoring the counting gas throughflow, it is also possible to regulatethe counting gas with the aid of such gas flow sensors in conjunctionwith pressure and temperature sensors.

Preferably the central counting anode and the high-voltage cathode inquestion are arranged relative to one another at a spacing of from 3 to13 mm, especially 5 mm, and can be operated at high voltages of morethan 1500 V. For that purpose, the plate-shaped electrodes must beinsulated from one another by insulating parts, such as the frame, thespacing pieces, and adhesion and casting compositions, those insulatingparts having high volume and surface electrical resistance values of,respectively, from 10¹² to 10¹⁴ Ω/cm³ and from 10¹⁶ to 10¹⁸ Ω/cm. Thisadvantageously reduces leakage or tracking currents, which wouldotherwise falsify the measurement and impair the sensitivity of thesystem as a whole.

In a further preferred embodiment, the ionisation chamber is developedto form an ionisation chamber system for ion beams. For that purpose, aplurality of ionisation chambers of the type according to the inventionare arranged behind one another in the direction of the beam and areused to form a system for monitoring the intensity of a heavy iontherapy beam. Owing to the high safety standards that must be met in thecase of therapy beams, at least two ionisation chambers are arranged intandem behind one another in the direction of the ionisation beam inorder to monitor the individual dose of a volume element and the layerdose of a scanned layer and are used for the independent monitoring ofthe total dose of a treatment cycle independently of the ionisationchamber for monitoring the individual dose and the layer dose.

Owing to the flat construction of a single ionisation chamber, thationisation chamber system has the advantage that in the direction of thebeam it requires very little space whereas, transverse to the beam, itextends over the full scanning surface. Since using a raster scanner theion beam scans the target volume by volume and by layer, advantageouslythe individual dose per volume element is monitored by a firstionisation chamber in the ionisation chamber system and the layer doseis monitored by adding together all the individual doses of a scannedlayer. A second ionisation chamber independent of the first ionisationchamber can advantageously monitor the total dose of a treatment cycle.Instead of the preferred two ionisation chambers, it is also possible toconnect three ionisation chambers behind one another, which then monitorthe individual dose of a volume element, the layer dose of a scannedlayer and the total dose of a treatment cycle, respectively.

In a further preferred embodiment, the first and second ionisationchambers monitor the pixel dose and the layer dose with redundancy. Thesecond ionisation chamber thus monitors the first ionisation chamber. Athird ionisation chamber is connected to different electronics andmonitors integral values, such as when the dose falls short of a maximumdose in the treatment plan.

In order to increase safety, the individual dose for a volume elementcan be measured in the first and second ionisation chambers of anionisation chamber system comprising three ionisation chambers, and theresults of the first and second ionisation chambers can be compared sothat if the measured data from the first and second ionisation chambersdepart from a predetermined tolerance range, a rapid switch-off of theion therapy system can be triggered. Such a comparison increases theoperating safety of the first and second ionisation chambers. Similarly,the layer dose of a layer to be irradiated (also called irradiationlayer) can be measured and compared by the second and third ionisationchambers of the ionisation chamber system comprising three ionisationchambers so that if the measured results from the second and thirdionisation chambers exceed a predetermined tolerance range a safetyswitch-off of the ion beam therapy system can be triggered.

The method of monitoring the intensity of a heavy ion beam by means ofionisation chambers or by means of the ionisation chamber systemcomprises the following method steps:

a) measurement of the intensity dose of an irradiation volume element ofa planned irradiation raster for an irradiation layer by means of afirst ionisation chamber;

b) monitoring of the measured value of the intensity dose of anirradiation volume element by a second ionisation chamber arranged intandem behind the first ionisation chamber;

c) comparison of the measured value of the first ionisation chamber withthe monitoring value of the second ionisation chamber and clearance forthe irradiation of the next irradiation volume element of a plannedirradiation raster of an irradiation layer when the two irradiationintensity values match within a predetermined desired value range;

d) emergency switch-off of the radiation treatment when thepredetermined desired value range is exceeded and readjustment of theintensity when it falls short of the predetermined desired value range;

e) repetition of the steps for the subsequent planned treatment layersuntil the volume of tissue to be irradiated has been fully scanned;

f) integration of all the measured radiation doses of the monitoringvalues in a third ionisation chamber, which is arranged in tandem behindthe first and second ionisation chambers, in order to monitor the totalradiation to which a volume of tissue is subjected during a treatmentcycle.

By means of that method, advantageously the number of particles or heavyions extracted per second from an accelerator and used as an ion beamfor tumour therapy is measured. The number of particles is subject tolarge temporal variations and must accordingly be measured in real timeduring irradiation directly by those ionisation chambers. The currentmeasured at the outlet of the chambers is proportional to the ion beamcurrent when the particle energy remains constant. In typical beamcurrents of the accelerator, the currents coming from the ionisationchambers are in the region of μA.

The response speed of the ionisation chambers is limited by the drifttime of the ionised counting gas molecules in the ionisation chamber andhas a delay constant in the order of magnitude of about 10 μs. Measuringelectronics convert the current from the ionisation chamber to aproportional frequency of pulses. For that purpose, voltage signals areproduced from the current signal and pulses are formed from the voltagesignal by means of amplitude-frequency conversion, the frequency ofwhich pulses is proportional to the voltage. Accordingly, a pulsecorresponds to a specific charge produced in the ionisation chamber, thecharge in turn being produced by a specific particle number of the ionbeam. The number of pulses produced is thus proportional to the numberof ions flowing through the ionisation chamber.

Accordingly, using the method it is advantageously possible to monitorthe intensity dose of an irradiation volume element, the intensity doseof a total irradiation layer and finally the intensity dose of a wholetreatment cycle. The method also comprises safety-relevant redundancy,in that in practice both the intensity dose of an irradiation volumeelement and the intensity dose of an irradiation layer are measured intwo ionisation chambers and the measured values are compared directlywith one another, so that in the event of unacceptable deviations anemergency switch-off of the system can be triggered. It is also possibleto add to each of the three ionisation chambers summation electronics sothat all three ionisation chambers can monitor the total dose of aradiation cycle of an irradiation volume simultaneously and in parallel.Thus, by means of the ionisation chambers according to the invention andin particular by means of the ionisation chamber system according to theinvention comprising three ion beam chambers arranged behind one anotherin the direction of the ion beam, it is possible to achieve the greatestpossible safety and reliability during the irradiation of a tumourvolume with ion beams.

In a preferred embodiment of the method, the order in which the methodsteps are carried out is adapted optimally to the planned treatmentcycle in question. In a preferred further development of the method, themonitoring function of the first and second ionisation chambers isprovided by a single ionisation chamber. Whilst this reduces theredundancy of the method, the space required for the detector systems inthe form of ionisation chambers is advantageously reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages, features and possible applications of the inventionwill now be explained in greater detail with reference to embodimentsand referring to the attached drawings.

FIG. 1 is a perspective view of the basic structure of a preferredembodiment of the ionisation chamber according to the invention.

FIG. 2 is a diagrammatic cross-section through the preferred embodimentaccording to FIG. 1.

FIG. 3 shows, in the Y-direction, the homogeneity of the local responsebehaviour of an embodiment of the ionisation chamber according to theinvention.

FIG. 4 shows, in the X-direction, the homogeneity of the responsebehaviour of an embodiment of the ionisation chamber according to theinvention.

FIG. 5 is a cross-section through the basic structure of an ionisationchamber system.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a perspective view of the basic structure of a preferredembodiment of the ionisation chamber 8 according to the invention. Theparallel-plate ionisation chamber 8 shown here serves to monitor andcontrol patient irradiation in the context of tumour therapy with heavyions. It consists essentially of a chamber housing 2, a beam inletwindow 3 and a beam outlet window 4, a chamber volume 5 filled withcounting gas, a high-voltage anode 6 and a high-voltage cathode 7 whichis arranged on both sides of the high-voltage anode 6. The ionisationchamber is intended for the field of medicine and is constructedsandwich-like and flat from plate-shaped large-surface-area structuresof the individual components, which are aligned orthogonally relative tothe ion beam 1. The components include a centrally arrangedlarge-surface-area orthogonally aligned plate-shaped counting anode 9,which is surrounded on both sides by a large-surface-area plate-shapedhigh-voltage cathode 7 consisting of two parallel cathode plates 10. Thechamber housing 2 consists substantially of a housing frame 11, whichframes a square ionisation chamber volume 12, and on which frame thebeam inlet window 3 and the beam outlet window 4 are mountedgas-impermeably. In the field of medicine, such an ionisation chamber 8must have a high level of safety, high reliability and must be very easyto maintain. For that reason the present embodiment meets the followingoutline conditions and requirements, the following irradiationparameters being determining for this embodiment:

Type of beam: protons, carbon ¹²C⁶⁺ (preferably) oxygen energy: 80 . . .430 MeV/u in 253 steps focus: 4 . . . 10 mm in 4 to 7 steps intensity: 2× 10⁶ − 4 × 10¹⁰ in 10 to 20 steps particles/extraction Geometry of 200× 200 mm² in the isocentre plane irradiation: Characteristic 1-10 s ionbeam extraction time times: 2-4 s beam pause ≧1000 μs per irradiationpoint of a volume element <1000 μs for interruption of the beam 100μs-250 μs per position measurement 10 μs-15 μs per intensity measurement

During an extraction phase of about 2 s, the carbon ions extracted froman ion accelerator system, such as a heavy ion synchrotron, fly throughthe ionisation chamber and on impact with the counting gas release someof their kinetic energy to that gas. Some of that energy results in theproduction of electron-ion pairs. The number of electron-ion pairs isproportional to the number of ions of the ion current that have flownthrough. The charges so produced in the active ionisation chamber volume12 are separated in an applied electrical field and recorded andevaluated by the downstream measuring electronics. For that purpose, theactive region consists only of the spaces between the electrodes. Onlycharges produced therein are detected and measured. The charges producedin the areas between electrodes and chamber windows are drawn off to thehigh-voltage planes 7 and/or to the windows 3, 4 and thus do notcontribute to the signal formation. They are excluded from themeasurement since the field inhomogeneities caused by the geometry inthose areas do not permit any representative intensity measurements.

In an ionisation chamber system 30 of three ionisation chambers 22, 23and 24, the evaluation takes place in two of those ionisation chambers22 and 23 every 12 its. By comparing those values with the desiredvalues determined previously in the context of radiation planning, thatinformation is used to regulate the scanning speed of a raster scannerat the site of treatment. The third ionisation chamber 24, shown in FIG.5, is of the same type of construction and is used to determine thetotal particle number for an isoenergy step, that is to say for anirradiation layer, and for the total radiation dose of a radiation cyclefor a tumour volume. By monitoring those integral values, namely theirradiation layer and the irradiation volume, further safety is providedfor scanning the tumour volume in a patient.

An area of about 190×190 mm² is covered by the scanning apparatus, ascanner, at the site of the ionisation chambers 22, 23, 24. Accordingly,the ionisation chambers 22, 23, 24 of that preferred embodiment have alarge active cross-section with an aperture width for the beam inletwindow 3 and the beam outlet window 4 of 210×210 mm². By using materialshaving a relatively high modulus of elasticity, it is advantageouslypossible to overcome the difficulties of such dimensions, especially inthe structure of the counter.

Each ionisation chamber 8, 22, 23, 24 is in the form of a Faraday cage,advantageously so avoiding electromagnetic interference. When cablingthe plate-shaped large-surface-area high-voltage electrodes 6 and 7,earth loops outside the ionisation chambers are avoided structurally inorder to minimise electromagnetic interference.

The housing frame 11 of the ionisation chamber 8 in that embodiment ismade from a solid aluminium material, the frames of the windows 18 aremade from stainless steel and the window films of the beam inlet window3 and the beam outlet window 4 are made of metallised plastics filmsthat are about 25 m thick, which are attached by adhesion in the frame18 of the windows so as to be electrically conductive. All conductiveparts that do not carry electrical potentials are earthed centrally bythe earth connection 25. Where possible, for materials through which theion beam 1 must pass in the active ionisation chamber volume 12 thereare used materials having low nuclear charge numbers, preferablyplastics comprising hydrogen and carbon that are as thin as possible andthus their mass present in the actual path of the ion beam is as smallas possible. This has the advantage that impairment of the beam quality,such as by projectile fragmentation and angular scatter of the beamparticles, is minimised by those materials. Moreover, onlyradiation-resistant and non-polarisable substances are used for suchmaterials in that embodiment.

The insulating parts, which are shown in FIG. 2, such as the frame 20 ofthe counting anode 6 and of the high-voltage cathode 7 and the spacingelements between those frames as well as the adhesion and castingcompositions have a volume and surface resistance value of,respectively, from 10¹² to 10¹⁴ Ω/cm³ and from 10¹⁶ to 10¹⁸ Ω/cm. Thosehigh surface and volume resistances have the advantage of reducing darkcurrents, which would falsify the measurements and impair thesensitivity of the system as a whole.

Conductive parts in that embodiment are provided with smooth surfaces,advantageously so avoiding high electrical field densities as triggerpoints for gaseous discharges. The films for the beam inlet window 3 andthe beam outlet window 4 are accordingly aluminised and the high-voltageanode 6 or counting anode 9 and the high-voltage cathode 7 or thehigh-voltage cathode plates 10 are accordingly manufactured from meshhaving a high mesh number and are coated with nickel, in order here alsoto achieve relatively smooth surfaces.

FIG. 2 is a diagrammatic cross-section of the preferred embodimentaccording to FIG. 1. According to the planar irradiation geometry, theionisation chamber 8, as shown in the perspective view of FIG. 1, is inthe form of parallel-plate ionisation chambers 8, 22, 23, 24. Theionisation chamber 8, as shown in FIG. 2, is symmetrical in theconstruction of its counting anode 9 between two high-voltage cathodeplates 10 with a spacing of 5 mm in each case in the direction of thebeam. That advantageously provides an exact definition of the activevolume. Furthermore, with the same drift time of the electron-ion pairs,the signal and thus also the signal/noise ratio is advantageously twiceas great as in an asymmetrical construction of, for example, two films.The non-active and thus dead zone of the ionisation chamber is from 8 to22 mm in the direction of the beam so that in the direction of the beamthe total dimension of the ionisation chamber 8 in that embodiment isfrom 18 to 32 mm.

The counting anode 9, spacing elements 21 and high-voltage cathodeplanes 10 are aligned with one another in order to obtain a precisedefinition of the active volume 12. For that purpose, those planes arearranged parallel to each other and perpendicular to the ion beam 1 bymeans of low-tolerance common suspension points of the planes in thedirection of the beam using corresponding spacing pieces.

Unavoidable leakage currents between high-voltage-carrying parts, suchas the cathode plates 10 and the counting anode 9, are kept away fromthe signal electrodes and conducted away by earthed protectiveelectrodes.

To produce the chamber planes, such as the cathode plates 10 and thecounting anode 9, in that embodiment a nickel-coated polyester mesh wasused. The chamber planes, especially the chamber electrodes, thus havehigh resistance to large ionisation densities as a result of highparticle densities and high particle rates in the ion beam 1 duringextraction from a synchrotron.

The nickel-coated polyester mesh of the electrodes preferably consistsof a screen print mesh having a fibre thickness of 38 μm, a fibrespacing of 54 μm and an open surface area of 34.45%. The fibre thicknessis composed of a 36 μm thick polyester core and a 1 μm thick nickelcoating. The average thickness of the nickel-coated polyester mesh is 62μm and the maximum equivalent range of the carbon beam particles in thatmesh is about 100 μm.

Both the use of mesh per se and the selection of polyester as carrierand nickel as coating material offer advantages that are describedhereinafter.

On the one hand, the use of mesh planes rather than films has thefollowing advantages:

a) the construction of the detector planes involved in the signalformation from mesh material permits greater mechanical pre-stressing ofmore than 10 Ncm compared with film planes.

b) the uniformity of the signal across the site is thus improved, whichhas a positive effect in particular in the case of the relatively largechamber cross-sections in the edge regions of the active volume of theionisation chamber 8.

FIG. 3 shows, in the Y-direction, the homogeneity of the local responsebehaviour of an embodiment of the ionisation chamber 8 according to theinvention, and FIG. 4 shows, in the X-direction, the homogeneity of thelocal response behaviour of an embodiment of the ionisation chamber 8according to the invention.

In FIGS. 3 and 4, the spacing in mm from the centre of the ionisationchamber 8 is plotted along the abscissa and the relative signalamplitude obtained is shown in % along the ordinate. The dotted line inthe upper region of FIGS. 3 and 4 symbolises the nickel-coated mesh ofthe electrodes 26 and 27, which is mounted in the frame 20 which has aninternal width w of 210 mm. It can clearly be seen that, over an activevolume breadth b of 190 mm, the local response behaviour of the chamber,that is the signal, is extremely homogeneous, and falls off steeply onlytowards the edge, that is say towards the mounting frames 20 of themeshes 26 and 27. That uniformity and homogeneity of the signal isespecially noticeable when the high voltage applied is large, at whichvoltage the planes of ionisation chambers comprising films already havea clear tendency to sag as a result of mutual electrostatic attraction.As a result of the higher mechanical pre-stressing of the meshes intheir frames 20, the sagging of such electrodes is less than in the caseof electrodes made from films. This is especially the case because,compared with film electrodes, the spacing of the electrodes from oneanother in the centre is hardly reduced at all and thus in meshelectrodes the field densities remain locally constant.

Triggering of sparks especially at the point of greatest sagging in thecentre of the ionisation chamber 8 does not occur until greaterhigh-voltages, at which voltages the metallisation in film chamberswould be destroyed in a very short space of time.

Comparatively this has the advantage of higher maximum operatingvoltages of about 1900 V for mesh chambers compared with about 1000 Vfor mesh chambers having the same electrode spacing made from filmelectrodes. Such elevated operating voltages have an advantageous effecton the charge collection behaviour of the ionisation chamber 8, sincethe drift times are reduced, advantageously so enabling a reducedresponse/reaction time of the system as a whole. That advantage isespecially important for safety reasons, for which a rapid interruptionof extraction is required in the event of an error.

Moreover, as a result of the higher mechanical stress and the greatermass of the mesh electrodes in that preferred embodiment, the naturalfrequency of the ionisation chamber 8 can be shifted to higherfrequencies. In ionisation chambers having mesh electrodes of thatembodiment, the frequency is about 500 Hz. That high frequency issubstantially more favourable than in ionisation chambers having filmelectrodes, in which even the smallest vibrations as a result of loudtalking or unavoidable structure-borne noise vibrations from the vacuumpumps can result in sometimes considerable falsification of themeasuring signal.

Thus, as a result of the mesh electrodes of the preferred embodiment ofthe invention, the microphonic effect that would otherwise causeinterference in ionisation chambers 8, 22, 23, 24 is avoided.

A further advantage of the mesh electrodes is the gas-permeability ofthat construction. As a result, the laminar flow of the counting gasthrough and between the high-voltage and signal planes is considerablyimproved, which also contributes to long-term stability. In addition tothose advantages, the use of metallised polyester meshes rather thansolid metal meshes offers the advantage of having less impact on thebeam since, owing to the smaller nuclear charge number of the compositematerial and the lesser mass present in the centre, fewer secondaryproducts or fragments are produced as the ion beam passes through themesh and also the angular scatter is less, which reduces the amount ofradiation to which healthy tissue is subjected in tumour irradiation ortumour therapy.

Owing to the high melting point and the excellent adhesion of nickelmetallisation, the nickel-coated mesh also has the advantage of greateroperating safety since damage caused by any sparks has less effect thanin the case of an aluminium coating. Thus, the good chemical resistanceof the nickel coating also reduces the aging effects as a result ofcracked counting gases, which occur when gaseous discharges aretriggered. The chemical resistance of the nickel or nickel coating thuscontributes to an increase in the length of use or service life of themesh electrodes used and thus to an increase in the service life of theionisation chamber.

The screen print mesh used in that embodiment is extremely inexpensivecompared with metal meshs, for example made of titanium. Also, screenprint meshes have a uniform quality, which also constitutes a greatadvantage for the manufacture of ionisation chambers 8, 22, 23, 24.

FIG. 5 is a cross-section through the basic structure of an ionisationchamber system 30. That ionisation chamber system 30 is composed ofthree ionisation chambers 22, 23 and 24, which correspond to theionisation chambers 8 shown in FIGS. 1 and 2, which are positioned onebehind the other in the direction of an ion beam 1. With those threeionisation chambers 22, 23 and 24 that are independent of one another,there is provided the redundancy in the beam current measurement that isnecessary to ensure safe operation.

The space available at the beam outlet in a radiation room in front of apatient is greatly limited. Accordingly, those ionisation chambers 22,23, 24 are installed to be slim (each of a depth of 35 mm) in thedirection of the beam in a compact construction. The ionisation chambersystem 30 is mounted on a side-mounted common base plate (not shown) sothat the base plate does not cover the detector surface or the beaminlet window 3 or the beam outlet window 4.

The ionisation chambers 8, 22, 23, 24 can be replaced individually onthe common base plate. For alignment with the centre of the beam, thecentral scratches on the housing frame 11 in question and on the windowsof the ionisation chambers 22, 23, 24 are brought into registration withthe system of co-ordinates created by a laboratory system in the medicalradiation room. The ionisation chamber system 30 as a whole ispositioned on the base plate by means of insulating justifying screws.The precision that can be obtained in this process is ±1 mm.

Arranged on each insulating chamber 8, 22, 23, 24 are adjustmentdevices, which ensure reproduction of the justification after anydevelopment of individual ionisation chambers 22, 23, 24. Each of theionisation chambers 22, 23 and 24 has brass threaded elements that areattached adhesively at two sites to be conductive and have atransmission resistance of less than 3 Ω. Good earthing of the system asa whole is obtained by means of those brass threaded elements.

The supply of high voltage to the symmetrically arranged high-voltagecathodes 7 is supplied to the high-voltage cathodes 7, which arearranged as cathode plates 10 symmetrically with the counting anode 9 ofthe active ionisation chamber volume 12, by way of correspondinghigh-voltage feed-throughs 32 in the housing frame 11. Monitoring of thehigh voltage by the sensor element 39, described hereinafter, alsoincludes detection of a break in a cable inside the chamber by measuringthe volume resistance between the connections 11 and the voltage divider(not shown) at the plane 10. The high voltage is stabilised by a filterdevice 31 having buffer capacitors 33, 34, as shown in FIGS. 1, 2 and 5,the filter device 31 being arranged in the ionisation chamber 8, 22, 23,24. By means of that filter device 31 in the ionisation chamber 8, 22,23, 24, signal variations and falsifications of the measurement valuescaused by varying operating voltages at high measuring currents areadvantageously avoided.

The initial resistance of the high-voltage supply is typically 10 MΩ.The time constant of the combination of high-voltage apparatus and cableplus ionisation chamber is in the order of magnitude of 1 ms, which issubstantially longer than the typical time constant of the extractionsubstructure of, for example, a synchrotron, which is a few 100 μs.About 10⁻⁸ As occur as collected charge in 1 ms. At a maximum voltagedrop below 1 V at the buffer capacitors 33, 34, a capacity of 10 nF issufficient. In the preferred embodiments of FIGS. 1, 2 and 5,high-voltage capacitors of 50 nF are arranged as buffer capacitors 33,34. In addition, the ionisation chamber 8, 22, 23, 24 of the embodimentaccording to FIGS. 1, 2 and 5 has in the chamber volume 5 a T-filter 35,which consists of two resistors 36, 37 each of 1 MΩ and a capacitor 38of, for example, 1.2 nF. That T-filter 35 has the advantage of keepinghum interference away from the high-voltage electrodes by filtering.

In order to measure and monitor the high voltage, the high voltage isobtained in the embodiment according to FIGS. 1, 2 and 5 by way of ahigh-voltage-resistant voltage divider having a fixed dividing ratio(for example 1:1000). Measurement of the operating voltage is effecteddirectly at the high-voltage cathode 7 by means of a high-voltage sensor39. Monitoring of the high voltage by means of the sensor 39 ispreferably used to monitor the charge collection efficiency of theionisation chamber 8, 22, 23, 24 since the recorded beam particle numberis dependent upon the size of the operating voltage.

In the embodiments according to FIGS. 1, 2 and 5, the ionisation chamber8, 22, 23, 24 has a special gas supply with counting gas of uniformquality and composition. Compared with air-filling, which because of theair humidity influences the sensitivity of an ionisation chamber andreduces the reproducibility of the measurements, a dry gas mixture isused as counting gas in this case. In the embodiment according to FIGS.1, 2 and 5, there is used as counting gas or chamber gas an argon orkrypton/CO₂ mixture of 80/20%. That gas mixture has the advantage ofhaving a sufficient W value as counting gas. The W value of that gasmixture is sufficiently low to obtain adequate signals when particlerates are low (for example a minimum of 10⁶ particles per second) andfor a good signal/noise ratio and is also sufficiently high to cover arelatively large dynamic range in the particle rates (e.g. factor 100,that is to say a maximum of 10⁸ particles per second).

The chamber volume 5 is hermetically sealed off from the environment sothat the counting gas can be kept pure because of the sensitivity of thesignal to contamination. Accordingly, even when the chamber is not inoperation, an exchange of gas as a result of diffusion with theenvironment cannot take place or can take place only to a minimalextent. The hermetic sealing of the chamber volume 5 is achieved bymeans of O-ring seals 19 of the beam inlet window 3 and of the beamoutlet window 4 relative to the housing frame 11.

Moreover, there are provided in the housing frame 11 self-closing gasducts, which are such as not to be confusable with one another, in thecounting gas inlet openings 13 and counting gas outlet openings 14 aswell as gas-impermeable electrical feed-throughs 32 and feed-throughs 45for the sensors incorporated in the housing frame 11. The window films40 of the beam inlet window 3 and the beam outlet window 4 are fixedgas-impermeably by adhesion in the frames 18 of the windows. Materialsthat do not releases gases are used for all the structural elements usedin the chamber volume 5.

The counting gas inlet opening 13 is arranged, with respect to gravity,at the bottom and the counting gas outlet opening 14 is accordinglyarranged at the top. Moreover, the ionisation chambers 8, 22, 23, 24have tubes with holes of different diameter so that, with theco-operation of those holes in the arrangement of the counting gas inletopening 13 and the counting gas outlet opening 14, a laminar flow can beachieved between the detector planes. A sufficient laminar flow ofcounting gas of more than two litres per hour through the ionisationchambers has the advantage of achieving reproducible measurement valuesthat are stable in the long term. Since the density of the gas mixtureused is higher than that of normal air, the special arrangement of thecounting gas inlet opening 13 and the counting gas outlet opening 14 isalso of importance in obtaining a laminar flow, since air cushions thatit would otherwise not be possible to remove would hinder a laminar flowand would reduce the purity of the counting gas.

In order to make it more difficult for the counting gas to becontaminated by the penetration of extraneous gases, the ionisationchambers 8, 22, 23, 24 are operated at a slightly elevated pressurerelative to the ambient air. The extent of the gas reflux from theionisation chambers 8, 22, 23, 24 is monitored by a sensor in the outletso that any leaks can be detected.

In the ionisation chamber 8, 22, 23, 24, sensors for gas pressure 15 andfor the gas temperature 16 are built into the chamber volume 5. Fromthose measurement values it is possible to determine directly the gasdensity, which is in turn used in the determination of the value of thebeam particle number. By means of the sensor for the gas pressure 15 andthe sensor for the temperature 16 it is thus possible to monitor thefilling of the ionisation chamber 8, 22, 23, 24 with counting gas and tokeep it constant or to take it into account in computer calculationsduring evaluation.

What is claimed:
 1. An ionisation chamber for ion beams (1) having a chamber housing (2), a beam inlet window (3), a chamber volume (5) filled with counting gas, a high-voltage anode (6) and a high-voltage cathode (7) wherein: the ionisation chamber (8) is constructed sandwich-like, from flat large-surface-area structures of the above-mentioned components, which are aligned orthogonally relative to the axis of the ion beam, wherein a centrally arranged large-surface-area orthogonally aligned flat counting anode (9) is surrounded on both sides by a large-surface-area flat high-voltage cathode (7) comprising two parallel cathode surfaces (10), and the chamber housing (2) comprises a housing frame (11) which frames a virtually square ionisation chamber volume (12); wherein a beam inlet window (3) and a beam outlet window (4) are mounted gas-impermeably and electrically conductively on the housing frame (11) in orthogonal alignment with the ion beam (1), the beam inlet window (3) and the beam outlet window (4) being metallised on the side facing the ionisation chamber volume (12) and the counting anode (9) and the high-voltage cathode (7) consisting of mesh comprising metal-coated plastics fibres and being mounted in frames (20) which are supported against the housing frame (11) in an electrically insulated manner, with electrically insulating spacing elements (21) defining the spacing between the counting anode (9) and the high-voltage cathode (7).
 2. The ionisation chamber according to claim 1, wherein the counting gas consists of a gas mixture of argon or krypton and carbon dioxide, preferably in a gas chamber volume mixing ratio of 4:1.
 3. The ionisation chamber according to claim 1, wherein the housing frame (11) has a self-sealing counting gas inlet opening (13) and a self-sealing counting gas outlet opening (14).
 4. The ionisation chamber according to claim 1, wherein the ionisation chamber (8) has sensors (15, 16) for the counting gas pressure (15) or the counting gas temperature (16) or the high voltage (3 9), which sensors are mounted in the housing frame (11) in gas-impermeably sealed openings (17).
 5. The ionisation chamber according to claim 1, wherein the beam inlet window (3) and the beam outlet window (4) comprises radiation-resistant non-polarised plastics films, which are secured to metal plate-shaped frames X18) which in turn seal off the ionisation chamber volume gas-impermeably from the beam inlet window (3) and from the beam outlet window (4) by means of O-ring seals (19) in the housing frame (11).
 6. The ionisation chamber according to claim 1, wherein the beam inlet window (3) and the beam outlet window (4) comprise polyimide or polyester films.
 7. The ionisation chamber according to claim 1, wherein the beam inlet window (3) and the beam outlet window (4) are aluminised or nickel-plated on the side facing the ionisation chamber volume (12).
 8. The ionisation chamber according to claim 1, wherein the counting anode (9) and the high-voltage cathode (7) comprises nickel-coated plastics mesh, preferably nickel-coated polyester mesh.
 9. The ionisation chamber according to claim 1, wherein by way of openings (13, 14) in the housing frame, the counting gas can be supplied into the region of the ionisation chamber volume that is lowermost with respect to gravity and can be discharged in the uppermost region.
 10. The ionisation chamber according to claim 1, wherein a gas flow sensor for monitoring the throughflow of counting gas is arranged outside the ionisation chamber volume (12).
 11. The ionisation chamber according to claim 1, wherein the ionisation chamber (1) can be operated, with a spacing of 5 mm between the high-voltage cathode (7) and the central counting anode (9), at a high voltage of more than 1500 V because of the spacing between the high-voltage cathode (7) and the central counting anode (9) and the composition of the counting gas of an argon/carbon dioxide or krypton/carbon dioxide mixture in a ratio of 80.20%.
 12. The ionisation chamber system for ion beams comprising of a plurality of ionisation chambers (22, 23, 24) of the type according to claim 1, wherein the system is used to monitor the intensity of a heavy ion beam, with at least two ionisation chambers (22, 24) being arranged in tandem behind one another in the direction of the ion beam in order to monitor the individual dose of a volume element and the layer dose of a scanned layer, and to monitor the total dose of a treatment cycle independently of the ionisation chamber for monitoring the individual dose and the layer dose.
 13. A method of monitoring the intensity of a heavy ion beam by means of ionisation chambers according to the following method steps: a) measurement of the intensity dose of an irradiation volume element of a planned irradiation raster for an irradiation layer by means of a first ionisation chamber (22); b) monitoring of the measured value of the intensity dose of an irradiation volume element by a second ionisation chamber (23) arranged in tandem behind the first ionisation chamber; c) comparison of the measured value of the first ionisation chamber with the monitoring value of the second ionisation chamber and clearance for the irradiation of the next irradiation volume element of a planned irradiation raster of an irradiation layer when the two irradiation intensity values match within a predetermined desired value range; d) emergency switch-off of the radiation treatment when the predetermined desired value range is exceeded and readjustment of the intensity when it falls short of the predetermined desired value range; e) repetition of the steps for the subsequent planned irradiation layers until the target volume to be irradiated has been fully scanned; and f) integration of all the measured radiation doses of the monitoring values in a third ionisation chamber (24), which is arranged in tandem behind the first and second ionisation chambers, in order to monitor the total radiation to which a target volume is subjected during a treatment cycle.
 14. The method according to claim 13, wherein the order in which the method steps are carried out is adapted optimally to the planned irradiation cycle in question.
 15. The method according to claim 13, wherein the monitoring function of the first ionisation chamber (22) and of the second ionisation chamber (23) is provided by a single ionisation chamber.
 16. The method according to claim 13, wherein the method is used to monitor ion intensities for ion beams. 